The focus of this work was on fathers across the transition to fatherhood. The purpose of theempirical studies was to explore the association of testosterone (T) and paternal investmentincluding interpersonal differences in personality traits and relationship quality. Providingpaternal care is associated with a reduced likelihood of engaging in competitive or matingbehavior and also of providing protection when necessary. T is a psychobiological marker ofreproductive behavior.Recent studies found decreasing T levels in males across the transition to fatherhood, inorder to reduce mating effort in favor of providing paternal care. In addition, there is empiricalevidence for reducing relationship quality in parents across the transition to parenthood.Therefore, a direct association of T level and relationship quality was suggested. Moreover, researchers have assumed that sensation seeking (SS) is associated with both mating effortand T. For this reason, the personality trait SS was included in further investigations of thesecond study of this work.Thirty-seven fathers and 38 men in committed romantic relationships without children(controls) were recruited. On two days (four weeks prior to (t1) and eight weeks after birth(t2) for fathers, and three months after the first measurement day for controls), all subjectsrepeatedly collected saliva samples for T measurement at three times of the day, filled in aprotocol of activities and completed online questionnaires.